1. Field of the Invention
The present invention relates to a high-gain photodetector of semiconductor material and to a manufacturing process thereof.
2. Description of the Related Art
As is known, silicon is currently the main material used for manufacturing integrated electronic components and it is used for implementing a large variety of electrical functions.
At present, a new optical-communication technology is emerging wherein the elementary information is carried by optical signals. Wavelengths for optical communications are in the 1.3 to 1.55 μm range. It is moreover desirable to combine optical and electronic functions in a single silicon device, by combining electronic technologies and optical technologies.
High-internal-gain detectors are required for different applications, such as single-photon counting and quantum computing. Avalanche photodetectors (APDs) with internal gain up to 105 are particularly suitable for the purpose. Silicon-based avalanche photodetectors can, however, operate only at wavelengths of less than 1 μm. However, in applications such as data transmission in an optical-fiber system, different wavelengths are required, as mentioned previously. For such applications, compound semiconductor-based avalanche photodetectors are therefore used, where the compound semiconductor materials are typically ternary compounds of In, Ga, and As, in so far as such materials present high absorption levels at these frequencies. One advantage of avalanche photodetectors lies in the fact that it is possible to completely separate the acceleration region (where the electric field is maximum) from the absorption region.
It has moreover been demonstrated that rare earth ions, incorporated into silicon in the trivalent state, have well-defined electronic transitions due to the presence of a non-complete 4f shell. For example, erbium incorporated in the trivalent state has a first excitation state at 0.8 eV (corresponding to 1.54 μm) with respect to the ground state. This transition energy depends upon the specific rare earth ions, and, for example, it is approximately 1.2 eV for ytterbium (Yb), 1.16 eV for holmium (Ho), and 1.37 eV for neodymium (Nd). These transitions may be excited both optically and electrically, using a charge-carrier mediated process.
For greater clarity, the optical-excitation process is described that occurs when a photon having an energy resonating with the transition energy of rare-earth ions produces excitation of the ion from its ground state to its first excited state. This process is schematically illustrated in FIGS. 1a-1c in the specific case of erbium. In FIG. 1a, a photon having an energy of 1.54 μm and incident on an erbium doped region, is absorbed by an erbium ion, which is excited. The excited erbium ion may subsequently get de-excited, transferring its energy to the electronic system of the semiconductor. In the example illustrated, the erbium ion, during de-excitation, releases its energy to an electron which is at the top of the valence band (energy EV) bringing it to a defect level ET in the silicon band gap (FIG. 1b). Next, it may happen that the electron that is in the defect level absorbs thermal energy so that it passes from the defect level ET to the conduction band EC (FIG. 1c). Altogether, in the process illustrated, absorption of a photon at 1.54 μm leads to the generation of a free electron-hole pair. This electron-hole pair can then be separated and attracted by the electric field present in the region accommodating the rare-earth ion, thus giving rise to an electric current which can be detected and which is directly proportional to the intensity of infrared light.
The process of conversion of infrared light into electric current described above has been demonstrated in silicon solar cells doped with erbium, for which photocurrents have been obtained having a wavelength of approximately 1.54 μm. However, in these cells the conversion efficiency is very low, of about 10−6, and is not sufficient for implementation in commercial devices.
European Patent Application No. EP-A-0 993 053 entitled “Infrared Detector Integrated With a Waveguide and Method for Manufacturing,” filed on Sep. 1, 1998, herein incorporated by reference in its entirety, describes a waveguide structure able to detect infrared light in a silicon detector and using the process described above with reference to FIGS. 1a-1c. 
Another mechanism for light detection mediated by rare earths occurs when a photon directly excites an electron which is at the top of the valence band (energy EV), bringing it to the defect level ET (FIG. 2a). Also in this case, subsequently it may happen that the electron that is in the defect level goes into the conduction band EC by absorbing thermal energy (FIG. 2b). Also in this case, therefore, absorption of a photon at 1.54 μm leads to the generation of a free electron-hole pair.